1. Field of the Invention
This invention relates generally to the formation of carbonaceous pitches particularly useful in the formation of shaped carbon articles, especially carbon fibers. More particularly, this invention relates to improvements in pitch composition, thereby rendering them more suitable for forming optically anisotropic pitches containing less than 25 wt. % quinoline insolubles.
2. Description of the Prior Art
It is well known that optically anisotropic carbonaceous pitches can be used to form a wide variety of carbon artifacts. One carbon artifact of particular commercial interest today is carbon fiber. Hence, although particular reference is made herein to carbon fiber technology, it will be appreciated that this invention has applicability in areas other than carbon fiber formation.
Referring now in particular to carbon fibers, suffice it to say that the use of carbon fibers in reinforcing plastic and metal matrices has gained considerable commercial acceptance where the exceptional properties of the reinforcing composite materials such as their high strength to weight ratios clearly offset the generally high costs associated with preparing them. It is generally accepted that large scale use of carbon fibers as a reinforcing material would gain even greater acceptance in the marketplace if the costs associated with the formation of fibers could be substantially reduced. Thus, the formation of carbon fibers from relatively inexpensive carbonaceous pitches has received considerable attention in recent years.
To date, all high strength, high modulus carbon fibers prepared from pitches are characterized in part by the presence of carbon crystallites preferentially aligned parallel to the fiber axis. This highly oriented type of structure of the carbon fibers has been obtained either by introducing orientation into the precursor pitch fiber by high temperature stretching of the pitch fiber or by first forming a pitch fiber which possesses considerable structure.
In forming the carbon fiber from the pitch material which has a high degree of orientation, generally it has been considered necessary to thermally transform the carbonaceous pitch prior to fiber formation, at least in part, to a liquid crystal or so-called mesophase state. This thermal transformation typically is achieved at temperatures of between about 350.degree. C. to about 500.degree. C. and over exceedingly long time periods. For example, at 350.degree. C., the minimum temperature generally required to convert an isotropic pitch to the mesophase state, at least one week of heating is usually necessary and then the mesophase content of the pitch is only about 40%, the balance being an isotropic material. At higher temperatures, for example at temperatures of about 400.degree. C., at least ten hours of heating are usually necessary to have complete conversion of the isotropic pitch to the mesophase state.
As will be appreciated, a wide variety of complex reaction sequences take place during the thermal treatment of isotropic pitches; and it is these reactions which result in the formation of large parallel aligned lamellar optically anisotropic molecules which are known as mesophase pitch. Indeed, studies have shown that when heating natural or synthetic pitches at temperatures in the range of about 350.degree. to 550.degree. C., small insoluble liquid spheres begin to appear in the pitch which gradually increase in size as the heating is continued over a period of time. Ultimately, the spheres begin to coalesce into large domains which display strong optical anisotropy characteristic of parallel alignment of the liquid crystal phase. This mesophase transformation has been followed quantitatively by polarized light microscopy investigations of solvent extracted samples of thermally treated pitches in which the untransformed isotropic matrix is dissolved in a solvent such as pyridine or quinoline and the insoluble mesophase fraction is recovered by filtration.
More recently it has been discovered that isotropic carbonaceous pitches contain a separable fraction which is capable of being converted very rapidly, indeed generally in less than about 10 minutes and especially in less than 1 minute when heated to temperatures in the range of from about 230.degree. to about 400.degree. C. to a strongly optically anisotropic deformable pitch containing greater than 75% of a liquid crystal type structure. This highly oriented optically anisotropic pitch material formed from only a fraction of an isotropic carbonaceous pitch has substantial solubility in pyridine and quinoline. Consequently, such material has been referred to as neomesophase pitch, the prefix "neo", which is Greek for new, being used to distinguish this anisotropic pitch material from mesophase pitches which are substantially insoluble in pyridine and quinoline. Basically, the neomesophase former fraction of pitch is isolated by solvent extraction of well-known, commercially available graphitizable pitches such as Ashland 240 and Ashland 260. The amount of neomesophase former fraction of the pitch that is separable, however, is relatively low. For example, with Ashland 240, no more than about 10% of the pitch constitutes a separable fraction capable of being thermally converted to neomesophase.
As indicated hereinabove, the amount of time to convert a carbonaceous isotropic pitch at elevated temperatures to the mesophase state is quite lengthy. On the other hand, the separable fraction of the carbonaceous pitch which is capable of being rapidly converted at relatively low temperatures to a deformable pitch that contains greater than 75% of an optically anisotropic material is relatively small.